1. Field of the Invention
The present invention pertains to a method for fracturing an earth formation to improve the injectivity of fluids through an injection well to enhance the production of oil and similar liquids from the formation.
2. Background
In oil fields which do not produce adequate flow of oil into producing wells, such as due to declining natural formation pressures, various stimulation techniques including water and other fluid injection processes are carried out. Typically, fluid injection wells are drilled at predetermined sites with respect to the producing wells so that the maximum sweep efficiency of oil or other formation fluids may be produced for a given amount of injected stimulation fluid. Additionally, as reservoir pressures decline certain selected production wells are converted to injection wells.
In order to maximize the amount of injected fluids at the lowest cost, certain formations require fracturing at the injection wells. However, conventional fracturing methods used for producing wells have been determined to not be suitable for many injection wells. For example, conventional practice for fracturing formations for producing wells includes mixing a proppant material, such as sand or other hard particulates, into the fracturing fluid so that the fractures or fissures opened in the formation are maintained in an open position after the fracture fluid pressure is relieved. In order to assure that the fracture does not close adjacent to the wellbore, enough proppant is injected to maintain proppant in the fracture at the near wellbore vicinity so that when fluid production commences, the fracture will not close or pinch off the flow of fluids into the well.
Moreover, to some extent it is undesirable to provide proppant in an injection well when the fluid injection pressure is greater than the formation fracture pressure because the presence of proppant in the fracture tends to interfere with the flow of injected fluids through the fracture and out into the formation. However, a completely unpropped fracture is not desirable since injection rates must be maintained at undesirably high levels to maintain fracture growth. Moreover, exposing a "clean" formation to injection fluids may be more easily accomplished in a propped fracture having relatively fixed dimensions than a fracture that is growing continuously from injected fluids.
The present invention provides an improved injection well fracturing method which overcomes the disadvantages and deficiencies of prior art practice including those mentioned above.